I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for providing wireless telecommunication service using a Code Division Multiple Access (CDMA) "over-the-air" interface in conjunction with a Global System for Mobile communications (GSM) A-interface protocol interface.
II. Description of the Related Art
The Global System for Mobile communications (GSM) wireless telecommunications standard is a set of widely available digital telecommunications protocols for use within a digital wireless telephone system. The GSM specifications were developed by an international effort and have been adopted by the European Telecommunications Standards Institute (ETSI, 06921 Sophia Antipolis Cedex, France). A wireless telephone system configured in a manner consistent with the use of the GSM standards is shown in FIG. 1. GSM mobile-services switching center (MSC) 16 switches or connects telephone calls between the wireless system access network, namely the base station subsystems (BSS) 15, and wireline based public switched telephone network (PSTN) 18, which may also be a public land mobile network (PLMN). GSM MSC 16 provides telephone switching, billing, subscriber unit tracking, subscriber unit authorization, and some handoff control functionality. BSS 15 is comprised of base station controller (BSC) 14 and any base transceiver station(s) (BTS) 12 coupled thereto. As defined in the GSM specifications, the interface between GSM MSC 16 and BSS 15 is referred to as the GSM "A interface," which separates the GSM network switching equipment from the time division multiple access (TDMA) based radio equipment. BSC 14 is involved with handoff processing and signal processing resource allocation within BTSs 12 so that multiple subscriber units 10 can conduct telephone calls simultaneously. BTS 12 interfaces the subscriber units 10 via radio frequency (RF) signals and a well defined "over-the-air" protocol to the GSM wireless network. BTS 12 comprises radio transmission and reception devices, up to and including antenna devices, and also all the signal processing specific to the radio interface. BTSs can be considered as complex radio modems. Subscriber unit 10 provides generic radio and processing functions to access the GSM network through the radio interface to either the user of subscriber unit 10 or some other terminal equipment, such as a facsimile machine or personal computer. A particular subscriber unit 10 may switch the BTS 12 with which it interfaces as its location changes, but can only communicate with one BTS at a given instant. Within this application, the capability to switch from one BTS 10 to another BTS 10, where only one radio interface exists at any instance, is referred to as subscriber unit hard handoff.
To make a wireless telephone call, a network connection must be established between subscriber unit 10, often referred to as a "mobile unit," and PSTN 18. PSTN 18 is the conventional wireline telephone system. To conduct the telephone call in a mobile fashion, a portion of the network connection is formed via the exchange of radio frequency (RF) signals between subscriber unit 10 and BTS 12. The remaining portion of the network connection is typically formed through wire based connections that pass through BSS 15 and through GSM MSC 16. In accordance with the GSM "over-the-air" protocol, which is one of the protocols that make up the GSM wireless telecommunications standard, TDMA technology is used to establish a set of channels within the above identified RF signals used to interface a subscriber unit 10 with a BTS 12. These channels are used to separate and distinguish the various sets of data associated with the various telephone calls being made at any given time. The various sets of data include user data which normally takes the form of digitized audio information, and signaling data which is comprised of the signaling messages used to orchestrate the processing of a telephone call.
At the time of the inception of the GSM standard, the use of TDMA within the GSM over-the-air protocol increased the efficiency with which the given radio frequency bandwidth could be used to conduct wireless telephone calls. Increasing the efficiency with which the available radio frequency bandwidth is used is desirable because only a limited amount of RF bandwidth exists, and the amount of bandwidth is usually the limiting factor as to the number of calls that can be conducted by a particular wireless cellular telephone system. Since the inception of the GSM wireless telecommunications protocol, however, other wireless technologies have been perfected that allow a greater number of telephone calls to be conducted in a given RF bandwidth. Since efficient use of radio frequency bandwidth is highly desirable, the use of these more efficient technologies is now preferred.
One prominent and widely accepted example of a more efficient wireless telecommunications technology is Code Division Multiple Access (CDMA) signal processing and the associated over-the-air IS-95 protocol adopted by the Telecommunications International Association (TIA, 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006). With CDMA modulation techniques, each user traffic channel consists of a carrier modulated by a different high speed binary sequence, thereby spreading the spectrum of the waveform. Sets of user traffic channels share the same wideband frequency spectrum allocation, and both user data and signaling messages are transmitted over a user traffic channel. Additionally, each CDMA based BTS transmits overhead control signaling channels that carry information to enable the subscriber unit to acquire and access the system. These overhead control channels are also modulated with a high speed binary sequence and combined with the user traffic channels to comprise one wideband RF signal. Each CDMA based BTS transmits the combined RF signal, referred to as the forward CDMA channel, and receives the combined RF outputs of a set of CDMA based subscriber units located within an associated coverage area, where these combined set of outputs are referred to as the reverse CDMA channel. The forward CDMA channel is the sum of the forward pilot channel, the forward synchronization channel, one or more forward paging channels, and many forward user traffic channels that are each modulated with a distinct channel code and are combined with a PN spreading sequence. The reverse CDMA channel is the sum of one or more reverse access channels and many reverse user traffic channels that are each modulated with a unique channel code and are transmitted with a specific PN spreading sequence.
CDMA based wireless communication systems also offer an improved method of handoff for subscriber unit mobility. A handoff procedure known as "soft handoff" is afforded by the ability to utilize a subscriber unit's RF signals at more than one CDMA based BTS. This "soft handoff" ability of subscriber unit 10 to simultaneously engage in multiple RF interfaces with multiple CDMA based BTSs 12 provides transmission path redundancy as subscriber unit 10 moves from one location to another, thereby decreasing the chances of a call being dropped and of voice samples being lost. Additionally, the IS-95 protocol provides higher quality telecommunication service when compared to GSM since the CDMA signal is less susceptible to fade and noise interference. A subscriber unit communicating in accordance with the IS-95 protocol also consumes less power than a subscriber unit communicating in accordance with the GSM over-the-air protocol because the use of extensive power control algorithms are included in the normal operation of a CDMA system. This reduced power consumption allows the life of a battery used to power an IS-95 compliant subscriber unit to be extended beyond that of a GSM compliant subscriber unit.
Many regions having already existing GSM cellular telephone systems are reluctant to provide CDMA cellular telephone service despite it many benefits, however. This is because the incremental performance improvement provided by a CDMA system may not be sufficient to justify the cost of providing a completely new CDMA cellular telephone system when a previously existing system is available. This situation is in contrast to a region in which an entirely new cellular telephone system is to be built, where a CDMA cellular telephone system is often less costly to implement and provides higher quality service than a GSM cellular telephone system. If a method and system for implementing a CDMA cellular telephone system that utilized some of the existing GSM cellular telephone system infrastructure were devised, however, the cost of providing CDMA cellular telephone service in a region with an operating GSM cellular telephone system would be reduced. If the reduction were sufficient, the incremental performance benefit provided by a CDMA cellular telephone system could be justified in a greater number of locations. This would allow subscribers of cellular telephone service located in those regions to also have the benefit of CDMA cellular telephone service, and therefore such a method and system for implementing a cellular telephone system would be highly desirable.